This invention relates to hot melt adhesive compositions wherein the base or main adhesive material is a modified intermediate or high DS starch ester.
Hot melt adhesives are 100% solid materials which do not contain or require any volatile solvents. They are solid materials at room temperature, but, on the application of heat, melt to a liquid or fluid state in which form they are applied to a substrate. On cooling, the adhesive regains its solid form and gains its cohesive strength. In this regard, hot melt adhesives differ from other types of adhesives which achieve the solid state through evaporation or removal of solvents or by polymerization.
The major advantage of hot melt adhesives is the lack of a carrier fluid which eliminates the need for drying the adhesive film once it is applied to the substrate. This elimination of the drying step overcomes hazards associated with solvent usage and also allows for faster production line speeds and lower costs. The ability to modify the properties of the adhesive composition with the addition of various resins, oils, waxes and other additives, makes hot melt adhesives commercially useful in a wide variety of applications.
Historically, hot melt adhesives have been based on synthetic and natural resins and waxes, particularly petroleum derived polymers such as polyethylene, ethylene-vinyl acetate, styrenic block copolymers, and polypropylene to name a few. These compositions are further tackified, plasticized, and reinforced with a variety of resins, oils and waxes which are derived from both petroleum and naturally occurring feedstocks such as wood, gum and tall oil rosin and terpenes. However, such compositions suffer from the cyclical price changes common to all oil derived materials, and also are generally very resistant to degradation once the articles employing them are disposed of.
Starches have been used as adhesives in aqueous systems for many years. In such systems, water which is needed for application of the adhesive is removed by drying or other means after the application to the substrate leaving the adhesive bond or film. Starch has also been used in hot melts but typically as a filler or extender as disclosed in U.S. Pat. No. 4,131,581 issued Dec. 26, 1978 to J. N. Coker. In another type application, starch has been used in hot melts as a pre-applied activator material which releases moisture into the subsequently applied hot melt, see U.S. Pat. No. 4,913,969 issued Apr. 3, 1990 to H. Schulte, et al. Dextrin glues are applied from water by means of heating solutions of the glue in water at temperature and pressure above atmospheric and applying a foam of this material as disclosed in U.S. Pat. No. 4,396,529 issued Aug. 2, 1983 to R. Price, et al.
The present invention stems from the growing movement away from petroleum derived raw materials to those derived from natural resources and as part of an effort to utilize raw materials which have demonstrated some level of degradation. Despite the use of starch in different adhesive applications, there still exists the need for a natural based starch material which can be used in hot melts as the base or main functional adhesive material and which is compatible with the other various components typically found in hot melt formulations. This has not been easy to accomplish since starch will normally not melt in the absence of water or solvent and it has been difficult to formulate a hot melt composition containing starch as the primary or base adhesive material.